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研究生:姚台煒
研究生(外文):Yau, Tai-Wei
論文名稱:超快光場的量測
論文名稱(外文):Measurement of Ultrafast Waveform
指導教授:汪治平汪治平引用關係
指導教授(外文):Wang, Jyhpyng
學位類別:博士
校院名稱:國立臺灣大學
系所名稱:光電工程學研究所
學門:工程學門
學類:電資工程學類
論文種類:學術論文
論文出版年:2000
畢業學年度:88
語文別:中文
論文頁數:140
中文關鍵詞:波形量測超快光學相位回溯雙光子吸收非線性干涉術非線性光傳播克爾透鏡鎖模自聚焦
外文關鍵詞:waveform characterizationultrafast opticsphase retrievaltwo-photon absorptionnonlinear interferometrynonlinear propagationKerr-lens mode-lockingself-focusing
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超短脈衝雷射是目前研究瞬態現象和動態性質最重要的工具之一,透過光放大器更能夠將脈衝的光能量提昇至數焦耳,使得光強度達到數十垓瓦(10^12 W),足以驅動許多極端非線性的物理機制。由於非線性物理機制與光場的波形息息相關,因此全盤瞭解脈衝光場將能有效地幫助我們研究這些機制,並期望能夠更進一步來掌握這些新奇的現象。本論文將詳述我們在測量超快光場上的貢獻:在初期的工作,我們複製一套全光場解析的測量系統,能夠分析出光脈衝在時間上強度與相位分布等所有的資訊。
這套系統應用相位回溯的技術,從脈衝以克爾效應(Kerr effect)所產生的頻譜圖(spectrogram)來計算回實際的光場分布。測量的頻寬涵蓋脈衝整個頻譜,達到數十垓赫茲(10^12 Hz),相當於幾個飛秒(femtosecond =10^-15 sec)的解析度。同時系統的架構與單發式自相關儀相容,能夠輕易地從現有的自相關儀改裝,是一套相當實用且完整的超快光場量測系統。目前分析光場所需的非線性訊號大部分仰賴晶體內多波混合(wave mixing)的機制來產生。然而使用晶體產生訊號光的方式存在諸多限制,
本論文的第二項工作就是利用半導體內雙光子吸收(two-photon absorption)產生非線性訊號的機制,配合以基因演算法為基礎的相位回溯技術來解析超快光場的分布。由於整套量測系統僅使用半導體的光偵測器,因此整個系統極有潛力能夠積體化,製作成單一晶片的超快光場量測系統。由於非線性效應會改變光脈衝在時間與空間波形的分布,因此單單分析時間上的波形並不足以全盤瞭解這些非線性的交互作用,本論文的第三項工作就是發展出一套能夠分析脈衝光場三維空間分布的技術。從這項技術我們測量直接從克爾透鏡鎖模雷射(Kerr-lens mode-locked laser)
輸出的光脈衝在時間與空間上的波形。從測量出的波形我們觀察出光脈衝在時間中心部份具有較小的光束寬度。這個現象驗證自聚焦(self-focusing)現象確實是克爾透鏡鎖模雷射主要的鎖模機制。自聚焦現象是高能量雷射脈衝在介質中傳播最常引發的非線性現象。由於缺乏適當的分析技術,目前對脈衝在介質內傳播的研究都停留在時間波形的分析。應用我們發展的三維光場量測技術,我們測量出高能量超短脈衝在透明介質內光場的演化。我們觀察到脈衝在時間中心部份與前後兩翼部份自聚焦的程度幾乎相同,並且不論入射光功率的大小,在數個瑞立長度(Rayleigh range)內光束皆束陷到20-25 um 附近。在另一個獨立的極化調制實驗,我們驗證這些現象皆導因於非線性折射率會隨入射光功率而飽和。對於氧化鋁晶體,非線性折射率會飽和在 dn=7*10^-5處。以簡單的飽和數學模型,我們計算出這項飽和的非線性折射率會造成光束束陷到20 um大小,說明在固態介質中自聚焦效應會被折射率飽和現象所平衡,使得光束不會無限制地塌陷下去;相反地,光束則會束陷在一恆常的寬度中。
Ultrashort pulse laser has played an important role in
nonlinear optics research. By virtue of optical amplifiers, ultrashort pulses can gain energy up to several joules to drive many extreme nonlinear physical mechanisms, extending the frontier of physics to a very interesting state. Since the nonlinear mechanisms highly depend on the structure of pulse waveform, characterization of ultrafast waveform has got an increasing importance to help us gaining an insight into the mechanisms behind; and furthermore, to use them. This thesis presents our contribution to the topic of characterization of ultrafast waveforms. In the early work, we duplicated a complete waveform measurement system, which is based on
"frequency-resolved optical gating". This measurement system recovers the pulse waveform from a specific spectrogram generated by optical Kerr effect. The measurement bandwidth covers the whole spectrum of the pulses, which means the bandwidth achieves tens of tera Hertz, that is, a resolution of
several femtoseconds in time. Because the system setup is compatible to an single-shot autocorrelator, it is easy to operate and re-build the measurement system from the present autocorrelator. Up to date, most characterization methods use optical nonlinearity to generate signals; they face the trade-off between sensitivity and bandwidth. In order to maintain the self-referencing advantage while reducing the bandwidth-sensitivity trade-off, in the next work, we developed a phase-retrieval nonlinear interferometry which use the two-photon absorption effect in semiconductor detectors to generate the
nonlinear signal. With waveform retrieved by genetic algorithm, this system features broad bandwidth, high sensitivity, and robustness. Since only electronic detectors were used, the whole system can be miniaturized into a single-chip ultrafast waveform analyzer. Owing to the high-intensity they produced, ultrashort pulses always experience the waveform distortion both in temporal and spatial domain due to nonlinear effect.
Therefore, it is not sufficient to study the nonlinear mechanisms with only temporal waveforms. The third work we have done is to develop a technique that analyzes the pulse waveform
in three-dimensional space. By this technique, we measure the spatiotemporal profile of a Kerr-lens mode-locked laser pulse.
The waveform shows a smaller beam size in the pulse center than the temporal wings. This observation gives a direct proof that the mode-locking mechanism of Kerr-lens mode-locked laser is self-focusing. In ultrafast optics, self-focusing is an omnipresent effect in nonlinear propagation. However, due to the lack of space-resolved experiments, what are the dominant effects that govern the self-focusing remains unclear. In this thesis we study the nonlinear propagation of self-focused femtosecond pulses with the three-dimensional phase-retrieval technique. For high-peak-power pulses, nearly uniform self-focusing and quasi-stable single-filament trapping to an universal beam diameter were observed. These phenomena can be explained by the saturation of the nonlinear refractive index change at dn=7*10^-5. The saturation is verified by an independent cross-polarization modulation measurement.
封面
目次
摘要
1緒論
1.1以頻率解析光閘分析脈衝波形
1.2全半導體光場分析儀
1.3測量三維空間上的光場
1.4自聚焦現象的研究
2單發式頻率解析光閘
2.1基本原理
2.2相位回朔計算
2.3實驗系統
2.4實驗數據與結果
2.5總結
3全半導式相位解析自相關儀
3.1相位回朔計算與基因演算法
3.2系統架設與訊號處理
3.3實驗結果與討論
3.4總結
4以交叉相關法測量三維光場
4.1基本原理
4.2相位回朔交叉相關
4.3實驗系統
4.4自聚焦鎖模機制
4.5結論
5自聚焦現象的研究
5.1超過臨界功率光脈衝在介質內的演化
5.2非線性折射率的飽和
5.3討論
5.4總結
6結論
參考文獻
附錄
[1]. For Frequency-Resolved Optical Gating, see R. Trebino, K. W. DeLong, D. N. Fittinghoff, J. N. Sweetser, M. A. Krumbuegel, and B. A. Richman, "Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating,"
Rev. Sci. Instrum. 68, 3277 (1997); and references therein.
[2]. For Phase-Retrieved Nonlinear Interferometry, see T.-W. Yau, Y. Y. Jau, C.-H Lee, and J. Wang, "Photodiode-based phase-retrieval ultrafast measurements," in Conference on Lasers and Electro-Optics, Baltimore, Maryland, U.S.A., May 1999, paper CWF21; and references therein.
[3]. For Three-Dimensional Waveform Measurement and Nonlinear Propagation, see T.-W. Yau, C.-H Lee, and J. Wang, "Femtosecond self-focusing dynamics measured by three-dimensional phase-retrieval cross correlation," published in J. Opt. Soc. Am B, August 2000; and references therein.
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